The present application generally relates to systems and methods for determining transmission power levels for optical signals communicated by optical network units.
A Passive Optical Network (PON) communicates data between an Optical Line Terminal (OLT) and one more Optical Network Units (ONUs) connected via an optical distribution network (ODN). PONs utilize a point-to-multipoint physical layer architecture, where a power splitter is often used to connect a multiplicity of ONUs to the OLT. In an ideal scenario, the signal power arriving at the OLT from each ONU would be equal, as it typically is not desirable to favor the communications of one ONU over another in this manner. In practice, this equal receive power is rarely the case. The path loss between the OLT and each ONU varies depending on the physical interconnect the signal traverses, being impacted by distance and the arrangement of the splitters on the PON. This difference in loss along the various pathways is known as differential path loss.
In order to help compensate for the differential path loss such that all the signals arriving at the OLT will be more nearly equal, ONUs can adjust their signal launch power. An ONU with less loss to the OLT will launch less power, and an ONU with more loss to the OLT will launch more power. This technique is known as power leveling.
Power leveling is particularly important in newer generations of PON, such as an NGPON2 (next-generation passive optical network 2), which increase the capacity of the PON by operating multiple wavelengths simultaneously on the ODN via a mechanism known as time and wavelength division multiplexing (TWDM). In such a case, multiple ONUs may be transmitting at the same time but at different wavelengths or channels. In a perfect world, the signals at different wavelengths would be totally independent of one another. In practice, a signal in a given channel will have spectral tails, with some amount of energy bleeding into other channels. So long as this undesired energy bleeding over is small relative to the actual desired signal for that channel, communications will not be hindered. However, when differential path losses cause the desired signal arriving at the OLT to be very weak relative to the undesired tails from other channels, communications may be impaired.
FIG. 1 depicts an exemplary power spectrum for the received power at an OLT for three channels. Due to differential power loss, the received power for channels 1 and 3 may be much greater than the received power for channel 2 such that the tails of channels 1 and 3 bleed over into the band for channel 2 and are significant relative to the received power of channel 2. Power leveling may be performed whereby the transmit power of channels 1 and 3 are reduced so that there is less of a difference in the received power of channels 1-3, thereby reducing the interference in channel 2 caused by the tails of channels 1 and 3.
Typically, the power leveling process occurs during the ONU activation process, which occurs when an ONU joins the PON. In one instance, the OLT might measure the received signal level from an ONU and issue directives to the ONU to adjust its transmit level until the signal arrives at the OLT within a desired or optimum power range. However, prior to such power leveling process, it is possible for the power level of the signals from the ONU to exceed the optimum power range and cause unacceptable levels of crosstalk into neighboring channels. In order to avoid such interference, it is possible to configure the ONU to initially transmit at a low power level, well below the power levels that might cause unacceptable levels of crosstalk, and iteratively increase its transmission power until the OLT is able to hear the ONU and acknowledge its transmissions. However, such an iterative approach introduces delays into the activation process while the ONU is attempting to communicate at power levels too low to be successfully heard by the OLT.